The first is to slow the car under braking, extracting kinetic energy.

The second is to place a load on the ICE, extracting energy that would otherwise go to the wheels.

The second method is applied in various ways.

1. At low speeds, typically at the beginning of straights, when part throttle is demanded the ICE creates more power than is demanded and the MGU-K extracts power from the crank to make the power at the wheel match what the driver is demanding.

2. At high speed, at the end of straights, the ICE continues at full power and is used MGU-K to reduce the power going to the wheels and charge the ES.

3. Sometimes, at the end of braking events at speed below which there is insufficient traction to generate 120kW from the MGU-K (around 100kph) the ICE drives the MGU-K to supplement the power coming from the rear wheels. This is probably contentious but I believe I observed this with Honda at Monza last year. It has some other potential benefits.

4. The MGU-K charges the ES by transferring energy indirectly through the MGU-H using its rotational inertia. This route was publicised by Honda. It is done at the end of straights as in 2 above, and is used to exceed the 2MJ restriction for MGU-K to ES.

Using these techniques the teams can always obtain 2MJ even when, as noted by @FPV GTHO, the available braking energy is often around 1MJ.

The MGU-H generates energymainly at full throttle where it is possible the best PUs are generatingvatv70kW. I would expect that they can also provide some power at part throttle. If a lap is 60 seconds full throttle the MGU-H might be expected to generate in excess of 4.2MJ.

So 2 from the K and 4.2 from the H pretty much matches the 30/70 split suggested above.

Of course in the race the teams probably choose not to use the methods that drive the MGU-K from the ICE because they increase fuel consumption in which case the K proportion would go down. But of course they also have the option to coast at the end of straights to save this fuel energy to be used at the beginning of straights where it is most valuable.

Fortune favours the prepared; she has no favourites and takes no sides.
Truth is confirmed by inspection and delay; falsehood by haste and uncertainty : Tacitus

The first is to slow the car under braking, extracting kinetic energy.

The second is to place a load on the ICE, extracting energy that would otherwise go to the wheels.

The second method is applied in various ways.

1. At low speeds, typically at the beginning of straights, when part throttle is demanded the ICE creates more power than is demanded and the MGU-K extracts power from the crank to make the power at the wheel match what the driver is demanding.

2. At high speed, at the end of straights, the ICE continues at full power and is used MGU-K to reduce the power going to the wheels and charge the ES.

3. Sometimes, at the end of braking events at speed below which there is insufficient traction to generate 120kW from the MGU-K (around 100kph) the ICE drives the MGU-K to supplement the power coming from the rear wheels. This is probably contentious but I believe I observed this with Honda at Monza last year. It has some other potential benefits.

4. The MGU-K charges the ES by transferring energy indirectly through the MGU-H using its rotational inertia. This route was publicised by Honda. It is done at the end of straights as in 2 above, and is used to exceed the 2MJ restriction for MGU-K to ES.

Using these techniques the teams can always obtain 2MJ even when, as noted by @FPV GTHO, the available braking energy is often around 1MJ.

The MGU-H generates energymainly at full throttle where it is possible the best PUs are generatingvatv70kW. I would expect that they can also provide some power at part throttle. If a lap is 60 seconds full throttle the MGU-H might be expected to generate in excess of 4.2MJ.

So 2 from the K and 4.2 from the H pretty much matches the 30/70 split suggested above.

Of course in the race the teams probably choose not to use the methods that drive the MGU-K from the ICE because they increase fuel consumption in which case the K proportion would go down. But of course they also have the option to coast at the end of straights to save this fuel energy to be used at the beginning of straights where it is most valuable.

Thank you henry, that answers my question perfectly. But one thing I do not fully understand is how the energy transfer through the mgu-h into the energy storage works.

.....But one thing I do not fully understand is how the energy transfer through the mgu-h into the energy storage works.

what Honda calls the 'extra harvest' cycle
the K loads the ICE for a few milliseconds
turning a pulse of some ICE energy into a pulse of electrical energy lasting a few milliseconds
this electrical energy is used in real time to motor the H machine, accelerating its rpm for a few milliseconds
the H machine then is switched to generate for a few milliseconds so decelerating back to its previous rpm
during the deceleration this generated energy goes to the DC line and so to the ES
this cycle is repeated many times per second
the extra harvest means that the K generates more than the 2 MJ but the system counts the 'extra' as H-generated

also it's likely that the K generates for a few milliseconds to help to quicken and improve upshifts
(and correspondingly motors for benefit during downshifts)
the Honda telemetry shows evidence of this upshift action, more prominent in the lower gears where the rpm drop is greater
(only when the generated energy is quite large will it show on the DC line (it does sometimes) - as explained below)

the form of any transfer of electrical energy from eg the MGU-H machine to the ES ....
the controller for the MGU-H has its own internal ('onboard') capacitive energy store necessary for its own functioning
electrical energy from generating will raise the onboard store voltage towards the DC line voltage (ES battery voltage)
when this voltage is reached the onboard store is 'full' and further generated energy 'overflows' via the DC line to the ES
iirc the MGCU (controller unit) is nominally common to the MGU-K and the MGU-H
the total onboard capacitive store is allowed to be 300kJ

some of the ICE energy taken by MGU-K generation can be regarded as 'free' of fuel cost ie it is genuine recovery
because at partial ICE power the ICE efficiency is sub-optimal, increasing ICE power by K generation will improve PU efficiency

Whereas this topic is quite interesting, it appears to be descending into a tit for tat "debate". Obviously some will interpret the rules in a manner that suits them, but that does not mean that they are always correct.
So, can we please calm down a bit?

I did not say "under braking is in the rules" What I did was expressing my opinion on this here discussion forum and subject (second try have been let through) From among some more posts which weren't. and I have no problem with anybody disagreeing with my expressed opinion. but apart from expressing my opinion on this here subject my last (second try post) I was actually quoting from F1 official site 12-14 April 2019.

I wouldn‘t expect the f1 site to be always correct and neither use it as valid source. Sometimes they overgeneralise to better explain it in laymens terms. What‘s written in technical directives and the rules is what counts.

That being said, yes you can harvest from the mgu-k under braking to prevent giving the energy away as heat. But you could also harvest from the mgu-k while either off throttle or not under full load.

Now, how would that work? Well, the driver controls the power at the wheels through the accelerator pedal. What happens inbetween doesn‘t matter. If for example the driver requests 50% power from the engine, the ecu could apply full throttle to the ice and harvest 50% through either mgu-k or -h. As long as the requested power arrives at the wheels, that‘s fine.

Tl:dr:
You may harvest through the mgu-k under braking, but you‘re not limited to this usage

I wouldn‘t expect the f1 site to be always correct and neither use it as valid source. Sometimes they overgeneralise to better explain it in laymens terms. What‘s written in technical directives and the rules is what counts.

That being said, yes you can harvest from the mgu-k under braking to prevent giving the energy away as heat. But you could also harvest from the mgu-k while either off throttle or not under full load.

Now, how would that work? Well, the driver controls the power at the wheels through the accelerator pedal. What happens inbetween doesn‘t matter. If for example the driver requests 50% power from the engine, the ecu could apply full throttle to the ice and harvest 50% through either mgu-k or -h. As long as the requested power arrives at the wheels, that‘s fine.

Tl:dr:
You may harvest through the mgu-k under braking, but you‘re not limited to this usage

Second try in answering:- So now I am being told that one way the 'K' can harvest without the brake pedal being used is "The driver requests 50% power (50% throttle) from the engine, and the ECU could apply 'full throttle' to the ICE and harvest 50% through either 'K' or 'H'. just imagine me posting such technicalities on here.